This invention relates to laser diodes and, more particularly to apparatus for transforming the shape of the diode beam into a more symmetrical cross-section, i.e., one having a better beam quality.
The laser beam from the linear emitting segment or xe2x80x9cstripexe2x80x9d of a diode laser is very much wider in the xe2x80x9cXxe2x80x9d direction (typically 100 to 200 xcexcm) than in the xe2x80x9cYxe2x80x9d direction (typically 1 xcexc). Most applications, however, require a beam that is more symmetrical in the X and Y directions. In addition to the differences in beam length, the beams exhibit different divergences in the X and Y directions. The divergence angle of the beam in the width ( xe2x80x9cslowxe2x80x9d or xe2x80x9cXxe2x80x9d axis) direction of the stripe is small (e.g. 10 degrees) compared to the divergence angle in the perpendicular (xe2x80x9cfastxe2x80x9d or xe2x80x9cYxe2x80x9d axis) direction (e.g., 20 to 50 degrees). It is therefore usually necessary to employ a cylindrical lens to first collimate the beam in the Y direction. Beam quality is a measure of the number of times the beam""s divergence exceeds the diffraction limit. In high power applications it is especially desirable to have beam qualities in the X and Y directions that are as equal as possible.
U.S. Pat. No. 5,825,551 discloses an approach to improving beam quality in which a plurality of reflecting surfaces divert the beams emerging from a stripe and reconfigure them into a beam having a xe2x80x9csquarerxe2x80x9d cross-section. Unfortunately, the beams travel different path lengths, which can be a considerable drawback when, for example, the slow axis of the diode bar or stack is not collimated. Another approach is taken in U.S. Pat. No. 5,986,794 which reshapes radiation from a laser diode bar using sets of parallel optical plates. One set of plates shifts partial beams horizontally, another set shifts them vertically. It would be extremely advantageous, however, to come up with a more compact and easy to align device and which carries out both horizontal and vertical beam shift using just one plate or series of plates, or preferably, with a largely monolithic device.
In accordance with the principles of the invention, in one illustrative embodiment thereof, a prismatic body exhibits a set of entry panes or chamfers arranged along the width of the laser bar that are tilted at different angles to the optical axis. Advantageously, the prismatic body may be comprised of a stack of flat optical plates, a first plate of which exhibits the set of tilted entry panes to the width dimension of the laser bar. The entry panes on one side of a central pane of the first plate deflect the entering rays upward and to one side of the optical axis into plates stacked above, while those on the other side of the central pane deflect rays downward and to the other side of the optical axis. Plates stacked above and below the first plate receive the deflected rays and convey them to respective exit panes that re-direct and assemble them into parallel beams segments, stacked one above the other. By choosing plate materials having appropriate indices of refraction n, beam path lengths L within the plates, and the tilt angles of the entry and exit panes, the shifts in the X and Y directions sx, sy, respectively needed to effect a desired beam quality (wherein all beams desirably have the same path length L), can be calculated using Snell""s law. The system of equations to determine the parameters in question is as follows:
sin(xcex81x)=nxc2x7sin(xcex82x)
sin(xcex81y)=nxc2x7sin(xcex82y)
Lxc2x7cos(xcex81yxe2x88x92xcex82y)xc2x7sin(xcex81xxe2x88x92xcex82x)=SX
Lxc2x7cos(xcex81xxe2x88x92xcex82x)xc2x7sin(xcex81yxe2x88x92xcex82y)=Sy
where xcex81x, xcex81y are the angles that an incident beam makes with the normal to an input pane of a plate in the X and Y directions, xcex82X, xcex82y are the angles within a plate, n is the refraction index of a plate, L is path length of the beam inside a plate, and sx, sy are the desired distances of translation along X and Y axis. In one illustrative embodiment, the parameter L is chosen to ensure equal paths for each partial beam, reasonable values are chosen for sx, sy, and the above equations are solved to determine the angular parameters.